A heat-sensitive recording method has many advantages in that no particular developing step is required, (2) if paper is used as a support, the recording material can have a quality akin to that of plain paper, (3) handling of the recording material used is easy, (4) the images recorded have high color density, (5) this method can be effected using a simple and cheap apparatus and (6) no noise is caused during recording. Therefore, heat-sensitive recording materials have recently enjoyed a markedly increasing demand, particularly for use with a facsimile or printer, and have come to be used for many purposes such as a pass, a label or a score card. Moreover, it has been desired to devise transparent heat-sensitive recording materials which enable direct recording with a thermal head in order to adapt them for multicolor development, or to make them usable for an overhead projector (hereinafter abbreviated as OHP).
A possibility of provision of such transparent heat-sensitive recording materials depends on a possibility of provision of a transparent heat sensitive layer, and it is easily estimated that a demand for heat-sensitive recording material will be enlarged if said transparent heat-sensitive recording material is really provided.
For example, in a case when a heat sensitive recording material is used for before mentioned label etc., usually a heat sensitive layer is coated on a support then required format such as a ruled line, a trade name, quantity etc., are thermally printed on the heat sensitive layer. In this case, however, a stain which is caused by unexpected coloring is occurred when an organic solvent etc. are adsorbed on the heat sensitive layer. Therefore, a protective layer comprised of a material which is not damaged by the organic solvent should be provided on the heat sensitive layer to prevent above mentioned stain. Moreover, on these labels water such as rain often adsorbs then water soluble printing ink can not be used. Therefore, a special ink which contains selected organic solvents should be employed, but in this case, these organic solvents are often not good for health and a manufacturing cost becomes higher, and moreover a blocking phenomenon is apt to occurs in a manufacturing process since a long time is necessary to dry the printed labels. These disadvantages are solved if it is possible to set a transparent heat sensitive layer on a support having desired matters in print. However, heat sensitive layer of a conventional heat sensitive recording material which can be recorded by thermal head is not transparent, then a desired transparency of the heat sensitive recording material can not be realized even if the conventional heat sensitive layer is provided on a transparent support.
A transparent heat sensitive recording material which is known so far can not answer to the above mentioned new needs, since the transparent heat sensitive recording material is a type which is used by contacting with original document then exposing the recording material to light; a temperature of an image part is increased by an absorption of infrared light by image part of original then the recording material is colored imagewisely.
As the concentration of our energies on solution of the above described defects, it has been found that when adopting a combination of colorless or light colored electron donating dye precursor and color developer as a coloring agents, and microencapsulating the former and dispersing the latter in a special conditions, a transparent heat sensitive layer can be obtained by coating a mixed solution of a solution containing the microcapsules and developer dispersed solution. It has also been found that a transparency of the heat sensitive layer can be adjusted by selecting a ratio of two refractive indexes; one refractive index is that of a component contained in the microcapsule and the other is that of oily component contained in the developer emulsion.